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Cooling rates of LL, L and H chondrites and constraints on the duration of peak thermal conditions: Diffusion kinetic modeling and implications for fragmentation of asteroids and impact resetting of petrologic types

Submitted by Alicia on Tue, 08/30/2016 - 08:32

Title of Publication:

Cooling rates of LL, L and H chondrites and constraints on the duration of peak thermal conditions: Diffusion kinetic modeling and implications for fragmentation of asteroids and impact resetting of petrologic types

We have carried out detailed thermometric and cooling history studies of several LL-, L- and H-chondrites of petrologic types 5 and 6. Among the selected samples, the low-temperature cooling of St. Séverin (LL6) has been constrained in an earlier study by thermochronological data to an average rate of ~2.6° C/My below 500° C. However, numerical simulations of the development of Fe–Mg profiles in Opx–Cpx pairs using this cooling rate grossly misfit the measured compositional profiles. Satisfactory simulation of the latter and low temperature thermochronological constraints requires a two-stage cooling model with a cooling rate of ~50–200° C/ky from the peak metamorphic temperature of ~875° C down to 450° C, and then transitioning to very slow cooling with an average rate of ~2.6° C/My. Similar rapid high temperature cooling rates (200–600° C/ky) are also required to successfully model the compositional profiles in the Opx–Cpx pairs in the other samples of L5, L6 chondrites. For the H-chondrite samples, the low temperature cooling rates were determined earlier to be 10–20° C/My by metallographic method. As in St. Séverin, these cooling rates grossly misfit the compositional profiles in the Opx–Cpx pairs. Modeling of these profiles requires very rapid cooling, ~200–400° C/ky, from the peak temperatures (~810–830° C), transitioning to the metallographic rates at ~450–500° C. We interpret the rapid high temperature cooling rates to the exposure of the samples to surface or near surface conditions as a result of fragmentation of the parent body by asteroidal impacts. Using the thermochronological data, the timing of the presumed impact is constrained to be ~4555–4560 My before present for St. Séverin. We also deduced similar two stage cooling models in earlier studies of H-chondrites and mesosiderites that could be explained, using the available geochronological data, by impact induced fragmentation at around the same time. Diffusion kinetic analysis shows that if a lower petrological type got transformed by the thermal effect of shock impacts to reflect higher metamorphic temperature, as has been suggested as a possibility, then the peak temperatures would have had to be sustained for at least 10 ky and 80 ky, respectively, for transformation to the petrologic types 6 and 4. Finally, we present a model that reconciles textural data supporting an onion-shell parent body of H-chondrites with rapid cooling rate at high temperature caused by impact induced disturbance, and also discuss alternatives to the onion shell parent body model.

Fig. 1. Two representative profiles for Mg/(Mg + Fe) ratio (Mg#) in coexisting ortho- and clino-pyroxene (Opx–Cpx) pairs. The 2-Px pairs are in St. Se´verin (SS: LL6) and Grossvenor Mountains (GRO: L6) meteorites. The backscattered electron images and traverse lines (red lines) corresponding to the profiles are shown on the right. The scale can be judged from the length of a profile on the left and that for a traverse on the BSE images (20 lm and 11.5 lm in the Cpx side of St. Se´verin and GRO, respectively). (For visualization of colors, please see the web version of the article.)